Electrical utilities employ watt-hour meters to track, among other things, customer energy consumption. A common type of watt-hour meter for residential use is the induction meter. An induction meter includes a disk that spins at a rate of rotation that is proportional to the amount of energy consumed by a residence or other facility. To this end, induction meters typically include a voltage stator, a current stator and a rotating disk. The voltage stator generates a magnetic flux responsive to the voltage signal on the power lines of the residence. The current stator generates a magnetic flux responsive to the current or load signal on those power lines. The voltage and current stators are positioned to impart their respective fluxes onto the rotating disk such that it rotates at a rate proportional to the amount of energy consumed. The rotating disk then drives mechanical and/or electronic registers that accumulate the rotations of the disk. The utility uses the information from the registers to determine energy consumption for specified periods of time.
Induction meters are well known. An important aspect of all meters, including induction meters, is the accuracy with which they measure energy consumption. Inaccurate meters can result in overcharging or undercharging customers, which is undesirable. Accordingly, industry and government regulatory bodies, both domestic and international, specify requirements for meter accuracy.
One important set of requirements relates to registration error of the meter. Registration error is the amount of variance of the disk rotation rate, or registration, from an expected disk rotation rate. Registration error is tested by observing actual registration of a meter under known load current conditions. Typically, registration is tested at several known load current conditions, and the meter must exhibit registration error within limits for each of the load current conditions. For example, specific tests are carried out at 3 amperes of load current at unity power factor, 6 amperes of load current at 0.5 power factor, 200 amperes at unity power factor, and 200 amperes at 0.5 power factor. If a meter does not fall within a required window for each test load, then the meter must be calibrated until it complies.
However, proper calibration of a particular meter to comply with registration requirements is not always easily achieved. While several methods of calibrating or adjusting registration are known, such methods may cause reduction of error under some test conditions but undesirably increase error under other test conditions. It is therefore desirable to employ one or more calibration methods that are capable of adjusting registration only under certain load current conditions. Such a calibration method could be used to adjust registration at load current conditions in which the meter is in noncompliance without affecting the meter registration at load current conditions in which the meter is in compliance.
Two common calibration methods that calibrate registration only under selected conditions are light load adjustments and phase lag adjustments. Such calibration methods are in common use.
The phase lag adjustment adjusts meter registration for load currents at 0.5 power factor without affecting load currents at unity power factor. The phase lag adjustment is typically embodied by a shorted turn of copper wrapped around either the voltage stator or the current stator. The shorted turn slightly alters the phase of the magnetic flux in either the current stator or the voltage stator by introducing an out-of-phase flux density component therein. The out-of-phase flux density component changes the overall phase of the flux generated in the stator. The change in stator flux phase typically alters meter registration in a manner that reduces registration error. The shorted turn typically does not, however, appreciably adjust registration at unity power factor because of the disparity of effect of slight phase angle changes at unity power factor and 0.5 power factor.
The light load adjustment is used to compensate for registration errors at low load current. Light load adjustments are well known and discussed, for example, in U.S. Pat. No. 4,782,286 to Coburn et al. and U.S. Pat. No. 4,423,375 to Ramsey Jr., et al. Those patents and other light load adjustment techniques employ magnetic and/or nonmagnetic material near the pole faces of the voltage stator to shade or adjust the overall flux paths through the rotating disk.
While both of the above methods provide registration adjustments for particular load current conditions, they can not adequately address all situations in which calibration is necessary. For example, it has been observed in some meters that registration at 0.5 power factor is often too low at 6 amperes and too high at 200 amperes. Such disparity is often caused by nonlinearities in the voltage and current stator cores. That disparity is not troublesome if the amount of error at 6 amperes and 200 amperes is tolerable. However, in some cases, the amount of error is not tolerable particularly in certain foreign countries in which the tolerances at 6 amperes and 200 amperes are relatively narrow.
The shorted turn method is inadequate to correct such a problem because the shorted turn method uniformly adjusts the registration of the meter at 0.5 power factor at both 6 amperes and 200 amperes. Thus, while the shorted turn may reduce registration at 200 amperes, it also undesirably reduces registration at 6 amperes. Analogously, the light load adjustment can not resolve such a problem because it uniformly adjusts meter registration for low magnitude load currents at both 0.5 power factor and unity power factor. As a result, while the light load adjustment may be employed to increase registration at 0.5 power factor and 6 amperes, it also undesirable increases registration at low load currents at unity power factor.
Accordingly, there exists a need for a calibration method and/or apparatus that adjusts registration in a non-uniform manner for 0.5 power factor test load current conditions. Such a calibration method would permit calibration of a meter such that it provides accurate registration readings of an induction meter at both low and high current at 0.5 power factor, without distorting the registration reading at unity power factor.